1. Field of the lnvention
This invention relates to the measurement of distances along transparent graduations on a scale by detecting alternating light and dark areas resulting from light passing through such graduations. More particularly, the invention relates to a device and method for detecting alternating light and dark areas in the form of an interference pattern resulting from light passing through transparent graduations along an opaque scale and through a grating having transparent graduations and arranged to move relative to the scale wherein the device comprises a plurality of paired light sources and light detectors and wherein the graduations of the scale and grating pass between the light source and the detectors.
2. Background of the Prior Art
In the prior art, a plurality of paired light sources and light detectors were utilized so that light could pass through transparent graduations in a scale between the light sources and light detectors. Commonly, an additional scale (a grating) was provided so that an interference pattern was formed by light passing through both of the scales to create alternating light and dark areas. A plurality of paired light sources and light detectors were used since increased measuring accuracy could be obtained because the plurality of light sources and light detectors could detect light passing through the transparent graduations in a phased relationship. For example, one of such pairs could receive light passing through transparent graduations, two additional light sources and light detectors could receive light which was partially cut off through the transparent graduations and one of the light sources and light detectors would receive essentially no light since an opaque portion of the scale covered the light source. The electrical signals generated by each paired light source and light detector therefor depended in magnitude and phase upon the particular position of the pair along the scale. Distances could therefore be approximated which were even smaller than the width of the graduations on the scale or the width of the light areas in an interference pattern formed by more than one scale.
Unfortunately, such measuring devices were exceedingly difficult to manufacture since the light sources and light detectors had to be closely aligned in relationship to the graduations on the scale in relationship to the alignment of the other light sources and light detectors relative to the scale. No simple means for making such alignment has been available in the prior art.
Interference patterns known to those skilled in the art include a Vernier fringe pattern, a Moire fringe pattern or a stepped (or phased) fringe pattern. A Vernier fringe pattern is formed when the distance between the transparent graduations on the scales and grating are at slightly different intervals; thus light only passes through both scales when transparent graduations on both are aligned. A slight movement of one of the scales relative to the grating will change the amount of light passing through the transparent graduations at that point since the transparent graduations will no longer be in the same alignment. In a Moire fringe pattern, generally the scale and grating have transparent graduations which are of the same spacing; however, the scale and grating are not in perfect horizontal alignment, thus the graduations of the scale cross the graduations of the grating in an "x" pattern. Light therefore only passes through the graduations where transparent graduations cross. Slight movement of the grating and scale relative to each other causes the location of the crossed graduations to change, thus setting up the Moire fringe. A stepped fringe is similar to a Vernier fringe except that rather than different spacings between each graduation on the scale and grating, groups of graduations are changed in relative distance simultaneously. Thus, whole groups of graduations may be aligned or misaligned by movement of the scale and grating relative to each other.
The present invention is directed to adjustment of phase and amplitude relationships in Vernier and Moire fringe devices and not in stepped fringed devices, where relative phase is not meaningful.
In one prior art device of the Vernier type manufactured by Bausch & Lomb known as the Acu-Rite.RTM. device, phase was adjusted by rotating the index grating with respect to the scale. This was accomplished by providing a mechanical adjustment between the carriage which holds the index grating and the carriage top which rides on the scale edge. The phase angle between sine wave outputs from alternate paired light sources and light detectors could not be corrected without adjusting the gain. Furthermore, such an adjustment sometimes caused a misalignment between the light sources and light detectors.
In other prior art systems, such as the Heidenhain LS-803 and LS-903 devices, phase is adjusted by tipping a single light source and condenser lens assembly with respect to the grating, scale and photocells. This adjustment changes the phase angle between the two sine waves of alternate light detectors, but also affects the gains. Again, any movement of the light source or lens will affect the amount of the light falling onto the photocells, thus reducing incident light upon the cells. In other devices, adjustable set screws were provided to adjust light passing from the source to the detectors. These adjustments cannot effect adjustment of phase without also affecting the gain.
A further prior art system has been disclosed in German Gebrauchsmuster G 83 20 135.1, published on Dec. 8, 1983. There, an arrangement is shown for adjusting the light source in a length and angle measurement device. Such an arrangement, however, is believed to relate to a phased or step grating where the grating has the same pitch as the scale. There is no suggestion in the reference for applying the teachings to a Vernier or Moire grating as in the present invention and to the problem of providing independent adjustment of gain and phase in such gratings.
Yet another prior art construction is disclosed in German Patent No. DE 3229846 C2. That patent also relates to a step or phased grating. Apertures are available to adjust the amplitude of the supplied light. This construction does not relate to the independent adjustment of phase without affecting adjustment of gain.
None of the prior art thus provided for mechanical adjustment to independently adjust gain and phase relationships between light sources and light detectors. In prior art devices for incremental distance measurement using optical transmission through transparent graduations, there were no means for adjusting the direction of light upon each of the detectors individually so that the phase between the output signals from the detectors as well as the linearity of each such signal could be aligned. Furthermore, no such device permitted the independent adjustment of the quantity of light incident upon each detector with the same member used for aligning the phase. Additionally, there was no device for accomplishing both the gain and phase adjustment easily and quickly and at low cost. Furthermore, no such means was provided which required a minimum of space to enable the device to be miniaturized.
An object of the present invention, therefore, is to provide means for adjusting the phase angle of each of a plurality of light sources and detectors individually and independent of gain so that the phase between detector output signals from each detector can be aligned.
Another object of the present invention is to provide means for independently adjusting the individual gain of each pair of light sources and detectors with the same member used for aligning the phase of pairs of light sources and detectors.
A further object of the present invention is to provide a device for accomplishing the adjustment of gain and phase of a plurality of light source and detector pairs by an arrangement which is low in cost, and which is quickly installed and adjusted.
A still further object of the present invention is to provide for adjusting the gain and phase of light source and detector pairs by an arrangement which requires a minimum of space to enable miniaturization of a transducer head containing the light source and detector pairs.